Honestly, I would aim for doing an hour so ride out for half and then turn around at the minimum and build up. I don't know what you mean by out of shape or how old you are, but an hour a day of exercise is pretty much a basic requirement IMO and IME. If you are really working with your HRM then make sure you know what your resting heart rate is and look at heart rate training info so you understand the whole point. Your high heart rate isn't that high so you might be in better shape than you thought. If you are sticking at that kind of exertion then an hour should be fine unless you are on some kind of heart meds. Also while you don't want to be stranded, you are working out, the general consensus out there now is really to dumb down exercise, you WANT to come home knackered, sweaty and out of gas. It is just riding a bike.